Melt the butter in a heavy medium saucepan over medium heat. Mix cornstarch and lemon is small bowl. Remove the pan from the heat and whisk in the sugar, lemon juice, zest and cornstarch mixt. Whisk in the yolks until smooth.

Return the pan to medium-low heat and cook, whisking constantly, until the mixture thickens, 5 to 6 minutes. To check if the curd is thick enough, dip a wooden spoon into it and draw a your finger across the the back of the spoon; your finger should leave a path. Dont let the mixture boil.

Immediately force the curd through a fine sieve into a bowl, using a rubber spatula. Let cool at room temperature, whisking occasionally. Or skip the sieving and pour into bowl.

Refrigerate, covered, until ready to use.